LEDs are typically driven by a current source. LEDs are usually characterized by the manufacturer as having a certain brightness level at a maximum rated current. Any current exceeding this maximum rated current may reduce the reliability of the LED or damage it. Accordingly, LED driver designers sometimes include a means for the customer to set the maximum current delivered to an LED, since the driver may be used with a variety of types of LED.
LED drivers also typically enable the user to control the brightness level of the LED by controlling the continuous current level or controlling the average current level. The average current level can be controlled by controlling the duty cycle of current pulses to the LEDs. When controlling the brightness using pulses of current, such as by pulse width modulation (PWM), the pulses are optimally the maximum rated current, or close to such current, so that at 100% duty cycle the maximum brightness is achieved.
FIG. 1 illustrates one type of LED driver as a packaged integrated circuit 10. Assuming the IC 10 can drive up to three LEDs 12, the IC 10 includes one current source per LED 12. Each current source is connected to ground, assuming the positive voltage supply is connected to the anodes of the LEDs. To set the maximum current through the LEDs when the LEDs are controlled to have maximum brightness, the user selects a certain resistor 14 value. The data sheet for the IC 10 contains a formula or table for correlating the resistor 14 value to the maximum LED current. One end of the resistor 14 is connected to a fixed voltage reference, which may even be ground, and the other end is connected to a pin 16 of the IC 10. The IC 10 includes other pins, such as for power, ground, and enable. The voltage drop across the resistor 14 during operation of the IC 10, determined at least in part by the resistor 14 value, is then used by the IC 10 to set the maximum current through the LEDs 12.
A second pin 18 of the IC 10 receives a control signal related to the desired brightness of the LEDs. The control signal may be an analog signal that controls the duty cycle of an internal PWM controller, or the control signal may be the PWM pulse train itself, or the control signal may control the continuous current applied to the LEDs 12 by some other method.
It is desirable to reduce the pin count of driver ICs, both for reducing the cost of the IC and for simplifying the customer's application of the IC.
FIG. 2 illustrates another prior art driver IC approach, similar to that shown in U.S. Pat. No. 6,836,081, where the values of external resistors 20, 21, and 22 are selected by the user to directly control the maximum currents through the LEDs (higher resistance value causes lower maximum current). A dimming control signal applied to pin 24 of IC 26 controls the continuous or average current through the LEDs for brightness control, as described with respect to FIG. 1. Although the technique of FIG. 2 requires the IC 26 to have only one pin for the current control, the technique has the drawback of requiring the user to employ three resistors in the output circuit, which incurs extra cost and board space penalties.
What is needed is a single pin technique for an LED driver IC, where the single pin is used to both set the maximum current through the one or more driven LEDs and variably control the brightness of the LEDs.